Interaction between CD40 and its ligand (CD40L) plays an important role in IgE isotype switching, by activating deletional switch recombination, and in allergic inflammation by mediating the activation of CD40+ inflammatory cells (macrophages, dendritic cells, eosinophils). We have previously shown that the glucocorticoid (GC) hydrocortisone (HC) synergizes with IL-4 to induce IgE isotype switching in human B cells. Our recent findings indicate that HC induces CD40L expression in human PBMCs and Jurkat T cells. HC mediated IgE isotype switching is inhibited both by soluble CD40 (sCD40) and anti-CD40L mAb, and PBMCs from patients with CD40L deficiency fail to synthesize IgE in response to HC+IL-4. Cyclosporin A (CsA) inhibits HC mediated inductions of CD40L expression and IgE isotype switching. We propose to 1. Analyze the induction of CD40L expression by GC. We will investigate a). the cellular targets of GC induction of CD40L expression; b). GC-mediated CD40L dependent activation of B cells and non B cells; and c). the mechanism of induction of CD40L expression by GC in target cells. 2. Examine the role of the glucocorticoid receptor (GR) and of Glucocorticoid Response Elements (GRE) in the induction of CD40L expression by GC. We will investigate a). the GR specificity of induction of CD40L expression and IgE isotype switching by GC; b). GC induction of CD40L expression and IgE isotype switching in cells with deleted or mutated GR; c). the domains in GR that are important for induction of CD40L expression; and d). the role of GREs in GC induction of CD40L gene expression. 3. Analyze crosstalk between GR/GRE and NFAT. We will examine a) induction of CD40L expression by GC in cells deficient in NFAT; b). the effect of GC on NFAT activation; c). functional interactions of GR and GRE with NFAT in the CD40L promoter; and d). physical interaction of GR with the NFAT complex. The results of the proposed studies have important clinical implications because GC are commonly used to treat allergic diseases and because induction of CD40L by GC secreted in response to stress may provide a mechanism of early defense against infection.